/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.result;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.function.Supplier;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.Ordered;
import org.springframework.core.ReactiveAdapter;
import org.springframework.core.ReactiveAdapterRegistry;
import org.springframework.http.MediaType;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.web.reactive.HandlerMapping;
import org.springframework.web.reactive.HandlerResult;
import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
import org.springframework.web.server.ServerWebExchange;

/**
 * Base class for {@link org.springframework.web.reactive.HandlerResultHandler
 * HandlerResultHandler} with support for content negotiation and access to a
 * {@code ReactiveAdapter} registry.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public abstract class HandlerResultHandlerSupport implements Ordered {

    private static final MediaType MEDIA_TYPE_APPLICATION_ALL = new MediaType("application");


    protected final Log logger = LogFactory.getLog(getClass());

    private final RequestedContentTypeResolver contentTypeResolver;

    private final ReactiveAdapterRegistry adapterRegistry;

    private int order = LOWEST_PRECEDENCE;


    protected HandlerResultHandlerSupport(RequestedContentTypeResolver contentTypeResolver,
                                          ReactiveAdapterRegistry adapterRegistry) {

        Assert.notNull(contentTypeResolver, "RequestedContentTypeResolver is required");
        Assert.notNull(adapterRegistry, "ReactiveAdapterRegistry is required");
        this.contentTypeResolver = contentTypeResolver;
        this.adapterRegistry = adapterRegistry;
    }


    /**
     * Return the configured {@link ReactiveAdapterRegistry}.
     */
    public ReactiveAdapterRegistry getAdapterRegistry() {
        return this.adapterRegistry;
    }

    /**
     * Return the configured {@link RequestedContentTypeResolver}.
     */
    public RequestedContentTypeResolver getContentTypeResolver() {
        return this.contentTypeResolver;
    }

    /**
     * Set the order for this result handler relative to others.
     * <p>By default set to {@link Ordered#LOWEST_PRECEDENCE}, however see
     * Javadoc of sub-classes which may change this default.
     *
     * @param order the order
     */
    public void setOrder(int order) {
        this.order = order;
    }

    @Override
    public int getOrder() {
        return this.order;
    }


    /**
     * Get a {@code ReactiveAdapter} for the top-level return value type.
     *
     * @return the matching adapter or {@code null}
     */
    @Nullable
    protected ReactiveAdapter getAdapter(HandlerResult result) {
        Class<?> returnType = result.getReturnType().getRawClass();
        return getAdapterRegistry().getAdapter(returnType, result.getReturnValue());
    }

    /**
     * Select the best media type for the current request through a content
     * negotiation algorithm.
     *
     * @param exchange                the current request
     * @param producibleTypesSupplier the media types that can be produced for the current request
     * @return the selected media type or {@code null}
     */
    @Nullable
    protected MediaType selectMediaType(ServerWebExchange exchange,
                                        Supplier<List<MediaType>> producibleTypesSupplier) {

        MediaType contentType = exchange.getResponse().getHeaders().getContentType();
        if (contentType != null && contentType.isConcrete()) {
            if (logger.isDebugEnabled()) {
                logger.debug(exchange.getLogPrefix() + "Found 'Content-Type:" + contentType + "' in response");
            }
            return contentType;
        }

        List<MediaType> acceptableTypes = getAcceptableTypes(exchange);
        List<MediaType> producibleTypes = getProducibleTypes(exchange, producibleTypesSupplier);

        Set<MediaType> compatibleMediaTypes = new LinkedHashSet<>();
        for (MediaType acceptable : acceptableTypes) {
            for (MediaType producible : producibleTypes) {
                if (acceptable.isCompatibleWith(producible)) {
                    compatibleMediaTypes.add(selectMoreSpecificMediaType(acceptable, producible));
                }
            }
        }

        List<MediaType> result = new ArrayList<>(compatibleMediaTypes);
        MediaType.sortBySpecificityAndQuality(result);

        MediaType selected = null;
        for (MediaType mediaType : result) {
            if (mediaType.isConcrete()) {
                selected = mediaType;
                break;
            } else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION_ALL)) {
                selected = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }

        if (selected != null) {
            if (logger.isDebugEnabled()) {
                logger.debug("Using '" + selected + "' given " +
                        acceptableTypes + " and supported " + producibleTypes);
            }
        } else if (logger.isDebugEnabled()) {
            logger.debug(exchange.getLogPrefix() +
                    "No match for " + acceptableTypes + ", supported: " + producibleTypes);
        }

        return selected;
    }

    private List<MediaType> getAcceptableTypes(ServerWebExchange exchange) {
        return getContentTypeResolver().resolveMediaTypes(exchange);
    }

    @SuppressWarnings("unchecked")
    private List<MediaType> getProducibleTypes(ServerWebExchange exchange,
                                               Supplier<List<MediaType>> producibleTypesSupplier) {

        Set<MediaType> mediaTypes = exchange.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
        return (mediaTypes != null ? new ArrayList<>(mediaTypes) : producibleTypesSupplier.get());
    }

    private MediaType selectMoreSpecificMediaType(MediaType acceptable, MediaType producible) {
        producible = producible.copyQualityValue(acceptable);
        Comparator<MediaType> comparator = MediaType.SPECIFICITY_COMPARATOR;
        return (comparator.compare(acceptable, producible) <= 0 ? acceptable : producible);
    }

}
